I can't buy apple care

And were you here https://www.apple.com/support/products/mac/ when you tried to purchase it? And are you connected to the Internet so they can run diagnostics?

If you just bought if Friday, and when you do the check coverage, they may not have your serial number in their system yet.

Apple Care for what device?

And how old is the device?

From Apple Store or from 3rd party?

Go here: https://checkcoverage.apple.com and see what the coverage currently is and when the warranty ends.